Since the length unit “meter” was defined as that “the meter is the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second” in the 17th General Conference of Weights & Measures in October 1983, the CIPM has recommended, in succession, twelve light radiation wavelengths as optical wavelength standards. Later, various optical measurement methods using a laser wavelength as a length “ruler” were widely applied in research areas such as metrology, information science, communication, astronomy and so on. As various optical measurement methods are often performed in the air and the value of laser wavelength in the air is closely related to the refractive index of air, the measurement accuracy of the refractive index of air becomes one factor so important that restricts the improvement of accuracy for various optical measurement methods which take the laser wavelength as the length standard.
Methods for measuring the refractive index of air are generally classified into two types, i.e. the type of indirect measurement and the type of direct measurement. The indirect measurement method calculates the refractive index of air with the Edlén equation after measuring the pressure, the temperature and the relative humidity of the air. As the Edlén equation is obtained under a standard condition of the air, the difference between the composition of the air in the measurement environment and that of the standard air may cause an error in the measurement result. Even though the refractive index of air could be further rectified by measuring the content of carbon dioxide in the air, the measurement accuracy of the method cannot be better than 3×10−8 due to the measurement uncertainties from various air parameter sensors. Therefore, in some circumstances requiring precision measurement of a large-range with high-accuracy (for example, the ratio of measurement accuracy to measurement range is less than 10−9), it is necessary to measure the refractive index of air directly.
The direct measurement for the refractive index of air is usually achieved by use of interferometry including the multiple-wavelength laser interferometry, the Rayleigh interferometry, the evacuation measurement, the Fabry-Perot interferometry, the dual-wavelength interferometry and so on. Interferometry for measuring the refractive index of air generally takes the refractive index of the vacuum for reference to measure the number of interference fringes produced by the optical path difference when the light travels over two optical paths with the same length L both in the vacuum and in the air, i.e. 2L·(n−1)=(N+ε)·λ0 (wherein n is the refractive index of air, N is the integer of the interference fringes, and ε is the fraction of the interference fringes). When using interferometry for measuring the refractive index of air, the measurement accuracy depends on the subdivision coefficient of the interference fringes, and the length L of the optical path both in the vacuum and in the air. For example, when the L is 100 mm, and λ0=633 nm, if it is desired to obtain a resolution of 10−9 for the refractive index of air n, it is necessary for the subdivision coefficient of the interference fringes to be K= 1/3150. This may complicate the structure of the measurement system with a high cost. Therefore, in the prior art, most of the measurement accuracies for the refractive index of air on the basis of interferometry can but reach up to 10−8.